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The proliferation of blood lymphocytes from nonpregnant, nonlactating heifers was comparable with that of nonpregnant cows in their first lactation. Both low and high levels of β-hydroxybutyrate and low levels of prolactin, but not isoproterenol, reduced the proliferative response of dairy heifers and cows in their first or second lactation.

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This in vitro study examined the ability of important immune modulators [β-hydroxybutyrate (BHB), cortisol, prolactin, isoproterenol and insulin] to influence the responsiveness of peripheral blood mononuclear cells (PBMC) from multiparous dairy cows 29 ± 2 days before and 14 ± 3 days after calving. The activation and proliferation of PBMC in response to the mitogen phytohemagglutinin was estimated by the oxygen consumption rate after 24 hr and the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5diphenyl tetrazolium bromide) method after 72 hr respectively. In early lactation, the presence of 2 compared to 0.5 mmol/L BHB reduced PBMC activation (p < 0.05) and proliferation (p < 0.10), and the presence of 0.7 compared to 0.2 ng/ml insulin enhanced (p < 0.10) PBMC proliferation. In dry cows, the presence of low concentrations of BHB and insulin and both concentrations of prolactin (20 vs. 300 ng/ml) and isoproterenol (70 vs. 130 ng/L) enhanced activation (p < 0.10), but not proliferation (p ≥ 0.10) compared to cultures with no modulator addition. The presence or absence of high or low concentrations of hydrocortisone (20 vs. 45 nmol/L) did not (p ≥ 0.10) influence the activation and proliferation of PBMC from dry and early lactating cows. It is tempting to speculate that in antepartum PBMC the modulators represented an energy source or positive extrinsic signals to use nutrients for the activation process. On the other hand, PBMC from postpartum cows are known to be exposed to a metabolic challenging endocrine background. Under such conditions, high BHB concentrations and high insulin concentrations seem to act as negative and positive signals for PBMC, respectively, to utilize nutrients for activation and proliferation.

culture assay) and the 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyl tetrazolium bromide (MTT) assay (72-h cell culture assay). Cows were classified based on the in vitro proliferative response of the PBMC measured postpartum in low (n = 6), medium (n = 5), and high (n = 6) responders. We found no interaction of state of lactation with responder group for feed intake, milk yield, efficiency, metabolic traits, and immune cell activation ante-and postpartum. However, after calving, low-responder cows produced less methane per unit of body weight and per unit of energy-corrected milk compared with the other cows. This might be indicative of a low rumen fermentation intensity. Low responders might therefore suffer from a lower availability of digestible energy in early lactation and not be able to sustain the shift from immune cell activation to proliferation. If so, the selection of environmentally friendly low-methane emitters could promote phenotypes with a compromised immune response in the critical early lactation.